555 IC

555 is one of the famous timer IC. It is widely used as timer, square wave generator, multi-vibrator, modulator,PLL and more…

Block diagram of 555 IC

 The main part of 555 IC consists of two comparators and a voltage divider network. The voltage divider network (three 5K resistors) divides the supply voltage in to two reference voltages: viz. 1/3 Vcc and 2/3 Vcc. Comparator compares the these voltages with its trigger (pin 2) and threshold (pin 6) inputs.

when threshol input of 555 is more than 2/3 Vcc out put of comparator 1 become high. Out put of comparator 1 is given to set input of the controll flipflop. Same way when the trigger input becomes less than 1/3 Vcc out put of comparato 2 becomes high causes to rest input of controll flipflop to high. The Q out put of the controll flipflop is given to the inputs of discharge transistor and the buffer amplifier

Functions of pins

PIN 1: Ground

This is the ground pin. All the voltages are measure d with respect to this pin

PIN 2: Trigger

The inverting input of the comparator 2 which is compared with 1/3 Vcc, it is nothing but trigger input brought out as pin no2. when the trigger input is slightly less than 1/3 Vcc the out put of comparator 2 goes high. This out put is given to reset input of RS flip-flop.

PIN 3: Out put

The Amplified and complimented out put of the flip-flop is taken as out put

PIN 4: Reset

This is an interrupt to 555 IC. When this pin becomes low, it stops the working of the device and makes it off. So for the usual operations of 555 IC , the pin 4 must be connected to high.

PIN 5: Control voltage

Inverting input terminal of the comparator 1 is provided as control voltage pin. The voltage divider network hold this point at 2/3 Vcc . this point is the reference level for comparator 1 to compared with threshold input. So that when a voltage is applied to this input the reference volt for comp 1 may change and thus we can vary the out put pulse width.

PIN 6: Threshold

This is the non inverting input terminal of the comparator 1. when the voltage at this pin is more than 2/3 Vcc the o/p of comparator 1 goes high. This is given to set input of RS flip-flop thus high at S sets the flip-flop.

PIN 7: Discharge

The collector of discharge transistor is provided through this pin. When the o/p of the flip flop is high transistor saturates and give a low resistance discharge path to the external capacitor

PIN 8: +Vcc

The supply voltage is provided for 555 IC is through this pin. It can be in between 4.5V to 18V

Applications

Mono stable multi

Pulse width modulator

Figure shows a pulse width modulator. It is basically a monostable multi vibrator,exept its pin no5 controll input pin is replased by modulating signal. Internally pin no 5 is the point where the threshold comparators referense point. we can change the 2/3 vcc reference voltage by applying a voltage to this pin and heve out put pulse width changes

Astable multi vibrator

Square wave generator

In a astable mutil vibrator using 555 IC as above 50% duty cycle ( square wave) can't obtained. the modified circuit used to obtain square wave out put is shown below

Bistable Multi

Bystable multi using 555 timer is very rarely used. if the trigger and threshold inputs are taken as set and reset inputs, it become act as bistable multi.